buy the cheapest one, if you want to use gbit ethernet go with cat6 (even though i've gotten it to work with other cables it's stupid to gamble if your buying a new cable anyway). As they say buy the cheapest cat6 cable. If cat6 is unavailable cat5e should work but if you can get cat6 go with that.
It's all the same really, the isolation is not something normal people has to worry about (there are extra isolated cables). Depending on where you live i'm sure there is a chain or something that sells stuff for cheap prices (alternatively go online and order it).
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